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Although published data on the safety and effectiveness of castor oil for the induction of labor are sparse, it is widely used for this purpose outside of medical settings, especially by American nurse-midwives. Its low cost, easy storage, and apparent safety contribute to its popularity. The results of 4 small clinical trials evaluating its safety and effectiveness for labor induction were contradictory.

This historical cohort investigated the safety and effectiveness of castor oil for induction of labor in women with an estimated gestation of more than 40 weeks based on ultrasound. Data were obtained from the hospital-based records of pregnant women who had attended antenatal clinics on the Thai-Burmese border between 2005 and 2007. The 612 women (18.1%) who delivered during the study period at a gestational age of more than 40 weeks were divided into 2 groups: those who were prescribed oral doses of castor oil (n = 205) and those who were not (n = 407). A Cox proportional hazards regression model was used to measure the effect of castor oil on the time to delivery. The Fisher exact test was used to compare proportions of women with various identified adverse outcomes. Primary outcomes examined in the safety analysis were maternal deaths and stillbirths. Other analyzed safety measures included fetal distress, meconium-stained amniotic fluid, uterine tachysystole, uterine rupture, abnormal maternal blood pressure during labor, Apgar score, neonatal resuscitation, postpartum hemorrhage, and severe diarrhea.

No significant difference was found in the time to birth of women who received castor oil and those who did not; the hazard ratio was 0.99, with a 95% confidence interval of 0.81–1.20. There were no maternal deaths, uterine ruptures, or other harmful effects on the mother or fetus associated with either group.

In this study, the use of castor was safe for both mothers and babies but the data provide no evidence that this agent is effective for induction of labor.